In a synchronous digital communication network, since the clock is a very important constituent part, a working manner in which a master clock and a slave clock are equipped simultaneously in the design of communication equipment is adopted. That is, in a network element node, there are two clock units, and these two clock units can be master and slave interchangeably. Under normal circumstances, a master clock board provides timing synchronization signals for the communication equipment, and once the master clock board breaks down or is forced to switch by someone, a slave clock board immediately replaces the master clock board to provide the timing synchronization signals for the communication equipment. When the master clock and the slave clock are switching, it is required to ensure that services are not affected, for example, during the switching, service transient interruption, code errors, etc. should not be caused. Accordingly, in any cases, when the master clock and the slave clock are switching, it must be ensured that the master clock and the slave clock are consistent; otherwise the equipment will produce code errors and even service interruption.
As to how the master clock and the slave clock keep consistent, FIG. 1 is a schematic diagram of a structure of an apparatus for aligning the phases of the master clock and the slave clock in the existing art, and as shown in FIG. 1, a single clock chip is used to lock the slave clock to the master clock. However, after the locking of the master clock and the slave clock, although their frequencies are the same, there is still certain phase difference. When the phase difference between the master clock and the slave clock reaches to a certain extent, and when the master clock board and the slave clock board are switching, some abnormal conditions such as service transient interruption and code errors may occur. If we want to reduce the phase difference between the master clock and the slave clock, devices with more excellent performance are required, and thus the costs of the apparatus for achieving master and slave alignment is bound to increase.